Interpreter pattern

In computer programming, the interpreter pattern is a particular design pattern. The basic idea is to implement a specialized computer language to rapidly solve a defined class of problems. Specialized languages often let a problem be solved several to several hundred times more quickly than a general purpose language would permit.

The general idea is to have a class for each symbol (terminal or nonterminal) in the language.

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Uses for the Interpreter pattern

* Specialized database query languages such as SQL.
* Specialized computer languages which are often used to describe communication protocols.
* Most general-purpose computer languages actually incorporate several specialized languages.

Examples

Java

The following Java example illustrates how a general purpose language would interpret a more specialized language, here the Reverse Polish notation. The output is: '42 4 2 - +' equals 44

import java.util.*; interface Expression { public void interpret(Stack s);} class TerminalExpression_Number implements Expression { private int number; public TerminalExpression_Number(int number) { this.number = number; } public void interpret(Stack s) { s.push(number); class TerminalExpression_Plus implements Expression { public void interpret(Stack s) { s.push( s.pop() + s.pop() ); class TerminalExpression_Minus implements Expression { public void interpret(Stack s) { s.push( - s.pop() + s.pop() ); class Parser { private ArrayList parseTree = new ArrayList(); // only one NonTerminal Expression here public Parser(String s) { for (String token : s.split(" ")) { if (token.equals("+")) parseTree.add( new TerminalExpression_Plus() ); else if (token.equals("-")) parseTree.add( new TerminalExpression_Minus() ); // ... else parseTree.add( new TerminalExpression_Number(Integer.valueOf(token)) ); } } public int evaluate() { Stack context = new Stack(); for (Expression e : parseTree) e.interpret(context); return context.pop(); class InterpreterExample { public static void main(String [] args) { String expression = "42 4 2 - +"; Parser p = new Parser(expression); System.out.println("'" + expression +"' equals " + p.evaluate());

ee also

*Backus-Naur form
*Domain specific languages
*"Design Patterns" p. 243